What is the recommended PSI for bicycle tyres?

Look at the side wall of your tyres and you will be able to read the minimum and maximum PSI or Bar you can inflate your tubeless tyres or inner tubes up to.

Try to maintain the correct inflation pressure in your tube to lower the risk of punctures. Pump your tube to the pressure within that range.  If you are aiming for speed, you can inflate your tube right up to the maximum PSI indicated as that will help to reduce the rolling resistance of the tyre.

For better grip, you can let out some air for a more comfortable ride, especially when it is raining. Remember not go below the minimum PSI range recommended as the inner tube can get pinched between the rim and the tyre which can cause punctures. Also, it will be harder to ride a bike when the tyre pressure is too low.

If you enjoy riding your bicycle downhill, during descents, use your brakes in an on-and-off fashion to prevent the rim from been heated up as that could expand the air in your tube and cause a tyre blow-off. That is a good reason not to over inflate your tube.

If you visit Schwalbe’s website, you will find a tyre pressure chart calculated based on body weight and tyre width.

At Silca there is a link to "Tire Pressure Calculator". You can click on it to enter the information required, which includes the terrain you ride on, in order to calculate the PSI you need to inflate your tyres up to.

Some companies supply their pumps with a PSI chart attached. Based on this chart from PumpKing, for tyre size 700X28C, it is safe to pump your tube right up to 73 PSI if you are not able to read the markings on your tyre. 

Over time you will discover the perfect tyre pressure based on your personal experience and adjustment.  To make adjustment, you can start at the lower end of the recommended range and gradually add a few PSI after each ride, up to the maximum PSI recommended.  You can then stick to the inflation that provides you the best comfort and less resistance during rides.